1. Field of the Invention
The present invention relates to a field effect transistor-type moisture sensor for detecting a variation of the gate operation of a field effect transistor due to a variation of humidity, by a moisture sensitive means formed in the vicinity of the gate insulating film of a MOS- or MIS-field effect transistor device. The electric conductivity of the moisture sensitive means varies with the absorption and the desorption of water vapor or moisture.
2. Description of the Prior Art
A field effect transistor (hereinafter, referred to as FET)-type sensor, which comprises an FET device incorporated with a sensitive means exhibiting an electric variation of electrostatic capacity, electric conductivity, electrostatic potential, etc., due to a physical or chemical interaction with the physical quantity to be detected, detects the said physical quantity as a variation of the gate operation of the said FET device. Taking advantage of the high input impedance and the amplifying function of the FET device, such an FET type sensor can exhibit a high output, even though its size is extremely small, and moreover it can be easily adapted to an electronic technology, and thus is advantageous in actual use. Especially, an FET type sensor, which is constructed in such a manner to have a sensitive means in the gate region of the FET device, is advantageous practically and economically since the FET device can be small and a number of devices can be formed on the same substrate. However, such an FET type sensor containing the FET device therein is inferior to an ordinary FET device alone in the operation stability of the FET device. It is also inferior to an FET device in the output stability and the reproducibility of the output characteristic. Depending upon the kind of the FET type sensor required, materials and production processes of the sensitive means are so different that the operation characteristic of the FET device can be remarkably varied. As compared with an ordinary FET device, a large amount of impurities and/or ions are apt to appear in the sensitive means or contamination may occur in the interface between the sensitive means and the gate insulating film during the formation of the sensitive means on the FET device, causing instability not only in the operation characteristic of the FET device but also in the output characteristic of the FET type sensor. Moreover, since the FET type sensor, which is designed to be used as an atmosphere sensor such as a gas sensor, a moisture sensor, etc., is exposed to an atmosphere, it will be contaminated by impurities in the atmosphere, causing variation and/or deterioration of the FET characteristic and/or deterioration of the sensor itself. Accordingly, an FET type sensor must suppress the influence of impurities and/or ions contained in materials of the sensitive means or impurities and/or ions contaminating the interface between the sensitive means and the gate insulating film during the formation of the sensitive means on the FET device and/or during operation of the FET device, thereby providing for a stable output characteristic over a long period of time. If such an FET type sensor is designed, a variety of sensors such as gas sensors, moisture sensors, ion sensors, biological sensors, infrared-ray sensors, etc., will be able to be produced in an FET type format. FET type gas sensors, moisture sensors, ion sensors and biological sensors cannot avoid direct interaction of the sensitive means with the atmosphere so that the device therein cannot be covered with a package, etc. Therefore, the above-mentioned problems deriving from the contamination etc., of impurities and/or ions from the outside must be solved for FET type sensors.
To solve these problems, a silicon nitride film having a small diffusion coefficient concerning ions, moistures, etc., has been used as a gate insulating film, or used to cover the surface of the FET device. The resulting FET sensors are, however, still inferior in the output stability over a long period of time.
In order to solve the above-mentioned problems, the present applicant has proposed an FET type sensor having a double gate-electrode structure which was disclosed in U.S. patent application Ser. No. 697,640 and British Patent Application No. 8503061.
As a moisture sensitive material wherein an electrical resistance or an electrical capacity varies depending upon a variation of humidity or water vapor in the atmosphere, there have been, for example, a moisture sensitive material having a sintered body of metal oxides such as iron oxide (Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4), tin oxide (SnO.sub.2), etc., or a metal oxide film; a moisture sensitive material having a hydrophilic polymer film or a polyelectrolyte; a moisture sensitive material having an electrolyte salt such as lithium chloride (LiCl); and a moisture sensitive material having a hygroscopic resin or polymer film in which conductive particles or fibers such as carbon are dispersed.
While a moisture sensor containing a metal oxide film or a hydrophilic polymer film generally has a wide moisture-sensitivity range, its resistance varies exponentially, responding to relative humidity in the atmosphere. A moisture sensor containing a metal oxide has an excellent heat resistance and responds rapidly, but it has a high temperature resistance coefficient. Especially, moisture sensors having a sintered body of metal oxides are inferior in reproducibility and/or interchangeability of the moisture sensitive characteristics thereof because the moisture sensitive characteristic depends upon the constituents of the sensor to a great extent. A moisture sensor having an electrolyte salt such as lithium chloride detects only humidity in a narrow range and if it is allowed to stand in a highly humid atmosphere for a long period of time, the electrolyte salt therein is eluted or diluted resulting in deterioration of the moisture sensitive characteristic of the sensor, and accordingly it cannot be used for the determination of high humidity. A moisture sensor having a hygroscopic resin or the like, in which conductive particles or fibers are dispersed, cannot detect a humidity in a wide range because it exhibits a steep variation of the resistance thereof in a highly humid atmosphere, while it is not sensitive to low humidity. Also a moisture sensor having a hydrophilic polymer film or a polyelectrolyte film is inferior in humidity resistance, water resistance and durability, while it is advantageous in that it operates in a wide moisture sensitive range, has a rapid moisture sensitive response, a simple structure, and is easily produced at low cost.
In order to solve the above-mentioned problems of the moisture sensitive material, the present applicant has proposed a moisture sensitive material containing a crosslinked cellulose acetate butyrate film which was disclosed in U.S. patent application Ser. No. 707,588.